Hello! Thank you for the reply. I came across your blog in my search for an answer to this question, and I thought I would try this forum before e-mailing you directly. :-)
So, they are called soft scene breaks. I'd like to put in a respectful request for support for them in eReader input. I hope this only requires a simple tweak, but I'm prepared for less favorable news.
I've noticed these scene breaks for years in eReader e-books but just thought of them as three centered asterisks until I checked yesterday and found that they are handled differently in different eReader versions. (I also found it interesting that older versions of the eReader software show additional TOC elements or levels that aren't supported in the final version of eReader for Windows; these elements were also ignored in my Calibre conversions to EPUB and MOBI.)
In any case, I am very grateful for Calibre's conversion capabilities. I gave it a try on a whim and the results far exceeded my expectations, to put it mildly. Many thanks to you!